5

®

PCM56

STABILITY WITH TIME AND TEMPERATURE

The parameters of a D/A converter designed for audio

applications should be stable over a relatively wide

temperature range and over long periods of time to avoid

undesirable periodic readjustment. The most important

parameters are Bipolar Zero Error, Differential Linearity

Error, and Total Harmonic Distortion. Most of the Offset

and Gain drift with temperature or time is due to the drift of

the internal reference zener diode. The PCM56 is designed

so that these drifts are in opposite directions so that the

Bipolar Zero voltage is virtually unaffected by variations in

the reference voltage. Both DLE and THD are dependent

upon the matching and tracking of resistor ratios and upon

was designed so that any absolute shift in these components

has virtually no effect on DLE or THD. The resistors are

made of identical links of ultra-stable nichrome thin-film.

The current density in these resistors is very low to further

enhance their stability.

DYNAMIC RANGE

The Dynamic Range is a measure of the ratio of the smallest

signals the converter can produce to the full-scale range and

is usually expressed in decibels (dB). The theoretical dynamic

range of a converter is approximately 6 x n, or about 96dB

of a 16-bit converter. The actual, or useful, dynamic range is

limited by noise and linearity errors and is therefore somewhat

less than the theoretical limit. However, this does point out

that a resolution of at least 16 bits is required to obtain a

90dB minimum dynamic range, regardless of the accuracy

of the converter. Another specification that is useful for

audio applications is Total Harmonic Distortion.

TOTAL HARMONIC DISTORTION

THD is useful in audio applications and is a measure of the

magnitude and distribution of the Linearity Error, Differential

Linearity Error, and Noise, as well as Quantization Error. To

be useful, THD should be specified for both high level and

low level input signals. This error is unadjustable and is the

most meaningful indicator of D/A converter accuracy for

audio applications.

FIGURE 3. Full Scale Range Settling Time vs Accuracy.

Settling Time (µs)

0.01

1.0

0.3

0.1

0.03

0.01

0.003

0.001

R

0.1

1.0

10.0

Current

Output

Mode

Voltage

Output

Mode

The THD is defined as the ratio of the square root of the sum

of the squares of the values of the harmonics to the value of

the fundamental input frequency and is expressed in percent

or dB. The rms value of the PCM56 error referred to the

input can be shown to be:

where n is the number of samples in one cycle of any given

sampling point. The THD can then be expressed as:

=

100%

This expression indicates that, in general, there is a correlation

between the THD and the square root of the sum of the

squares of the linearity errors at each digital word of interest.

However, this expression does not mean that the worst-case

linearity error of the D/A is directly correlated to the THD.

For the PCM56 the test period was chosen to be 22.7

µs

(44.1kHz), which is compatible with the EIAJ STC-007

specification for PCM audio. The test frequency is 991Hz

and the amplitude of the input signal is 0dB, –20dB, and

–60dB down from full scale.

Figure 4 shows the typical THD as a function of output

voltage.

Figure 5 shows typical THD as a function of frequency.

(1)

∈

1/n

E

L

(i )

+ E

Q

(i)





2

i

= 1

n

∑

1/n

E

L

(i )

+ E

Q

(i)





2

i

= 1

n

∑

(2)

THD

=∈

FIGURE 4. Total Harmonic Distortion (THD) vs V

X

V

–60

10.0

0dB = Full Scale Range (FSR)

–60

–50

–40

–30

–20

–10

0

1.0

0.1

0.01

0.001

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